Assumptions of Physics

Introduction by field of study

Physics

There has always been an effort to formalize a physical theory from a small set
of well defined physics principles or laws. Newtonian mechanics, special relativity and thermodynamics,
for example, follow that pattern. Unfortunately, this practice has stopped in modern times.
Lagrangian mechanics and Quantum mechanics, for example, simply
posit the mathematical structure without a clear justification as to what is physically described
and why those rules exist. Physics is recovered at a later stage, by "interpreting" the results
of the mathematical theory.

In this project we want to revert back to that original spirit and put the physics at the center of our physical theories.
We want to identify those physical starting points, our physical assumptions, and then derive
the mathematical framework as a necessary consequence. By proceeding in this fashion, it is
always clear under what conditions a particular theory can be assumed to be valid, it is clear
what mathematical structure are physically meaningful and what are just artifacts introduced
by mathematicians to make calculations or proofs easier to develop. It also allows us to think
more deeply about our starting points, leading to new insights and possible new approaches.

Math

The foundations of mathematics settled during the beginning of the 20th century. Work by
Cantor, Russel, Zermelo, Fraenkel, Godel and many others re-organized the whole discipline
on top of logic and set theory. During that period physics was tackling new ideas in
both theory and experiment, which lead to general relativity and quantum mechanics, and
never really took that step at fully identifying the formal foundations of its discipline.

In this project we establish a general mathematical theory for experimental science,
which aims to axiomatize the underpinning of all scientific disciplines in the same way
that logic and axiomatic set theory establishes the underpinning of all modern math.
The core concept is the idea of an experimentally verifiable statement: an assertion that,
if true, can be verified to be so in a finite amount of time. We establish the logic of
these objects and we see how they lead to topologies and sigma-algebras. A physical theory,
then, is simply a collection of verifiable statement with a well defined semantic relationship
between them.

Uncovering the assumptions of physics means understanding how a physical assumption
is formally captured by the semantic relationship between statements, and how those relationships
necessarily lead to a given mathematical structure.

Philosophy

Over the last century, there has been an increasing interest within philosophy of
science on the foundations of the physical theories. Particularly in quantum mechanics,
the effort is to better characterize the mathematical structure describing the theory
in terms of "real" ontological entities. The main issue here is that the same mathematical
equation can be used to describe different physical systems (e.g. linear systems are used
in electronics, thermodynamics, biology, etc...) and the same physical system can be
described by different mathematical structures (e.g. a massive point particle can be modeled
by a point in a cotangent bundle or by a point in a tangent bundle).

In this project we develop the mathematical structures of science starting by clearly defining the physical
objects we are studying, with particular attention to the philosophical aspects that
arise with those definitions. As we do this from the very beginning, we are forced
to clarify many aspects that are usually "swept under the rug". As everything else is
built on top these definitions, the mathematical structures have one clear meaning and
there are no issue of interpretation. This provides a better framework to understand
what science is, what the laws of physics are and what they can or cannot be describing.

Overview of the framework

In this table we list the core ideas of our work and how they translate into the mathematical framework.

Our general mathematical theory of experimental science. A scientific theory is a collection
of statements that can be experimentally tested, which ultimately determine the possible cases that can be experimentally
distinguished. It must keep track of what can be verified (a topology), what can be predicted (a sigma-algebra) and a
way to establish the precision of different statements (a measure).

I

Assumption of determinism and reversibility. Given the present state of the system under study, all future and past states are uniquely identified.

Dynamical system. The state of the system is the
finest description that can be determined experimentally. Deterministic and reversible evolution preserves what
can be tested experimentally (the topology), what can be predicted (the sigma algebra), the precision of statements (the measure)
and the nature of the system (e.g. vector space, metric, ...). That is: it is a self-homeomorphism in the category (i.e. a continuous
transformation for the topology, a measure preserving map for the measure, a linear transformation for a vector space, ...).

II.a

Classical assumption of infinitesimal reducibility. The system under study is made of a
homogeneous material that can be decomposed into infinitesimal parts. Giving the state of the whole system
is equivalent to giving the state of each infinitesimal part. The evolution of each part is deterministic
and reversible.

Classical Hamiltonian particle mechanics. The state of the
composite system is identified by a distribution of the material over the states of the infinitesimal parts. The state space of the
infinitesimal part must allow to define the distribution independently from what coordinate system is used (a symplectic manifold).
Deterministic and reversible evolution can only move the density from one state to another without changing it and must
conserve the number of states as they evolve. That is: each infinitesimal part evolves according to Hamilton's equations
(a symplectomorphism).

II.b

Quantum assumption of irreducibility. The system under study is made of a
homogeneous material that can be decomposed into infinitesimal parts. Giving the state of the whole system
does not provide any description for the infinitesimal parts. The evolution of the system as a whole is deterministic and reversible
but not at the level of each infinitesimal part.

Quantum Hamiltonian particle mechanics. The state space of the homogeneous material
is identified by a distribution within which each infinitesimal part moves chaotically. This random motion can be described in terms two
independent stochastic variables (a complex distribution). While the overall system can still be assembled/disassembled into parts (a vector space),
the correlation between the random variables will result into constructive and descructive interference. Deterministic and
reversible evolution will preserve the total amount of material (unitary evolution) and therefore evolves according to the Schroedinger equation.

III

Kinematic equivalence. For the system under study, the kinematics (i.e. trajectories in physical space-time)
and the dynamics (i.e. trajectories in state space) are equivalent. Giving a set of initial conditions, such as position
and velocity, is equivalent to giving the initial state.

Lagrangian particle mechanics with
scalar/vector
potential forces. As we must be able to re-express the distributions defined over state variables as distributions over kinematic variables,
ranges of initial states have to map linearly to ranges of initial conditions (symplectic form induces a metric). This constrains the possible relationships between velocities and
momentum and limits the system to massive particles moving under forces described by scalar or vector potentials.